Method for coating two elements hybridized by means of a soldering material

ABSTRACT

This method of coating the area of hybridization of a component constituted by two elements associated with one another by means of a soldering material, comprises depositing in proximity to said component a coating substance capable of filling by capillarity the volume defined between the hybridized elements of said component. On the periphery of the hybridization area on the lower element of said component is defined an area ( 10 ) of non-wettability as regards said substance, by deposition of a covering anti-wetting as regards said substance, applied by PECVD, and defining on said first element, in line with the hybridization, a wetting surface for the coating substance.

FIELD OF THE INVENTION

The present invention relates to the microelectronics field, and to be more specific to that of techniques of hybridization by means of bumps of two heterogeneous components, also known as soldering bumps. It is connected therefore to the general field of micro-components, more conventionally denoted electronic chips, but also capable of being applied to micro-sensors, micro-actuators, such as Micro Electro Mechanical Systems (MEMS), and also to optoelectronic components, such as Vertical Cavity Semi Conductor Emitting Lasers (VCSEL).

Under the invention, the term component is taken to mean an electronic sensor, such as an electronic chip, an electrical or electronic circuit support, or a passive electrical component such as a protective cover, or even a physical quantity sensor.

PRIOR ART

The micro-components concerned in the context of the present invention are conventionally deposited on a substrate of an appropriate type, for example the semi-conductor type (single crystal silicon, sapphire, etc.) for electronic components.

Said substrate is provided with electrically conductive tracks, made out of aluminium for example, which radiate from the micro-component towards the periphery of the substrate, in order to make it possible, not only to supply to the component any electrical power that might be required, but also and above all to process the operation of the signals that said component is called upon to generate, and even to control the functions it incorporates.

To join different components to each other, one of the techniques in widespread use today is that of flip-chip hybridization by means of balls or bumps. This technology comprises in brief:

-   -   depositing onto one or more wettable surfaces, placed on one of         the components for joining, the material constituting the         welding bumps in an appropriate quantity;     -   providing the other component for hybridizing with surfaces that         are wettable by said welding material, said surfaces being         provided substantially in line with the surfaces of said first         component when the second component is transferred onto the         first;     -   depositing a flow of material presenting in liquid form, said         flow performing a chemical function of deoxidation and         prevention of reoxidation during soldering, a thermal function         enabling the transfer of heat, and a physical function enabling         surface tensions to be reduced and thereby promoting the         formation of the bumps themselves;     -   then, bringing the wettability surface of the other component         into contact with the soldering material so deposited;     -   and finally, raising the temperature until a temperature is         reached higher than the fusion temperature of the material         constituting the bumps in order to obtain the fusion thereof         until the required result is achieved, namely the hybridization         of the first component on the second component, said bumps         creating a mechanical and/or electrical link between the         wettable surfaces of each of the components, one at least of         said wettable surfaces being themselves connected to the         conductive tracks provided thereon.

Thus, during the assembly process, the soldering material constituting the bumps takes on the shape of a ball.

Because of the great number of components capable of being hybridized on a single substrate surface, particular care must be paid to the problem of the mechanical strength of the circuits so produced. In order to optimize said mechanical strength, there is a known technique of coating the electronic components so obtained after hybridization.

A prior art coating technique has in fact been shown in relation to FIGS. 1 and 2.

This coating method may comprise depositing a calibrated drop 5 of coating substance, such as a coating resin, onto the substrate 3 or read circuit of a component 1, in proximity to the hybridization of a chip 2 on said substrate.

Said calibrated drop 5 of resin is deposited near to the electronic component hybridized by means of bumps, said drop of resin migrating by capillarity under the electronic component or chip 2 added by hybridization. In other words, the resin will occupy the volume defined between the electronic component and the substrate and come to surround the hybridization bumps 6. The coating 7 is thus defined, guaranteeing the mechanical strength over time of the component so obtained after hybridization.

Said coating technique does however encounter a certain number of difficulties.

In the first place, in order to implement the capillarity effect, ensuring the effective migration of the coating resin, the drop of resin must be deposited sufficiently close to the component for hybridizing in order to generate a surface tension effect underpinning the capillarity.

In the same way, the volume of resin must be large enough, to fill completely the space defined between the component 2 and the substrate 3. However, said quantity must not be too excessive either, in order to prevent said resin from spreading beyond the substrate, which might, were this to happen, prevent the hybridization of other components in proximity.

An improvement to this coating technique has been described, for example in the document FR 2 704 691. In brief, this comprises positioning a carpet of balls in proximity to the bumps intended to hybridize the component on the substrate, said coating resin being deposited onto said carpet of balls. The latter ensures the migration of the resin by capillarity towards the hybridized component, thereby resolving the problem inherent in the proximity in a straightforward way.

However, this improvement does not resolve the problem of controlling the spread of the coating resin beyond the hybridized component, and an excess of resin is always able to spread onto the substrate. To escape from this difficulty, a proposal has also been made for the implementation of a seam of adhesive peripheral relative to the component, said seam acting as a barrier, the coating resin being deposited inside said seam.

However, this technique also comes up against various drawbacks among which may be cited the fact that the method implemented is carried out in two stages, which is detrimental to the manufacturing time.

Moreover, implementing said seam proves to consume surface on the substrate, thereby reducing the capacity for bringing together and concentrating the components on a single substrate.

The invention specifically sets out to find a solution for these different problems.

DISCLOSURE OF THE INVENTION

The invention thus applies, initially, to a method of coating the area of hybridization of a component constituted by two elements associated one with the other by means of a soldering material.

Said method comprises depositing in proximity to said component a coating substance, also known as underfill resin in the technical field under consideration, capable of filling by capillarity the volume defined between the hybridized elements of said component.

According to the invention, on the periphery of the hybridization area on the lower element of said component is defined an area of non-wettability as regards said substance, by deposition of a covering that is anti-wetting as regards said substance, said deposition being applied by Plasma Enhanced Chemical Vapour Deposition (PECVD).

In doing this, said non-wettability area provides a barrier relative to the coating substance, thereby avoiding any risk of dispersion of the coating substance over said lower element, and in general terms over the substrate on which said component is hybridized.

According to the invention, the anti-wetting covering, which is not very thick, is constituted for example by SiOC or C₄F₈.

According to one advantageous feature of the invention, a continuous surface is defined on said first element or on the substrate, in line with the hybridization area which is required to be coated, and intended to receive said coating substance.

Said area, wetting as regards said coating substance, is defined by default, in the sense that a coating that is non-wetting as regards the coating substance is applied on the periphery thereof.

According to one feature of the invention, said zone includes a first extension, located contiguous and in continuity with said wetting area, and intended to act as reception surface for the drop of coating substance.

According to another feature of the invention, said area also comprises a second extension, also contiguous and in continuity therewith, and intended to collect the excess coating substance, and located to advantage opposite to said reception extension relative to the hybridized component it is required to coat.

In other words, in these two designs, the non-wettability area, which by definition are peripheral to the hybridized component, has one or two extensions to define an area for receiving the coating substance and an area for collecting said excess coating substance respectively.

According to one particular feature of the invention, more specifically intended for optoelectronic components, a surface of non-wettability relative to the coating substance is also defined on the first element or on the substrate on which the second element is hybridized, located in the hybridization area and in a particular area in which the presence of coating substance is not required.

Said design may be found in the context of optoelectronic components in respect of which it may be required to hybridize a laser, the substrate then being bored with a through orifice, to let the laser beam through and to receive an optical fibre.

According to another objective of the invention, the specified method can be used to confer wettability properties afresh on the anti-wetting coating so obtained. This wettability feature inversion is obtained, in compliance with the invention, by subjecting the anti-wetting covering to the action of a plasma or to the action of an ultra-violet radiation. To this end, after the elements constituting the component have been hybridized together by means of the so-called “underfill” resin, in accordance with what has been said above, the assembly is coated with another resin, known in the field under consideration by the term “glob-top”, which confers excellent mechanical cohesion on said component.

In other words, this reversibility of the wettability properties enables components previously hybridized by “flip chip” to be coated by means of a coating resin or “glob-top” with good adhesion, and the electrical contacts under the chip or under the hybridized element have been coated with an underfill resin in a controlled way using deposition barriers that are hydrophobic, and therefore, not able to be bonded effectively.

Indeed, on depositions left hydrophobic, a “glob-top” coating would have poor adhesion, promoting incipient cracking.

It will be useful to recall that wettability reduces when the hydrophobic character increases.

DESCRIPTION OF THE FIGURES

The way in which the invention can be implemented, and the resulting advantages, will become clearer from the following embodiment examples, given for information purposes and non-restrictively, supported by the appended figures.

FIGS. 1 and 2 are, as already said, diagrammatic representations of the prior art coating principle.

FIG. 3 is a view from above of the method of coating a hybridized component in accordance with the invention.

FIG. 4 is a view from above similar to FIG. 3 of an alternative.

FIG. 5 shows the implementation of a coated hybridized optoelectronics component in accordance with the inventive method seen in cross-section, FIG. 6 being a view from above.

FIG. 7 is a diagrammatic representation of a hybridized component in accordance with the invention, and implementing a linear laser array and a linear lens array in the context of an optronics component.

FIGS. 8 a, 8 b and 8 c set out to show the reversible character of the wettability properties of the coating applied in accordance with the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

A description is given, in relation to FIGS. 3 and 4, of a method of coating a component obtained after hybridization. This is based fundamentally on the principle of the migration of a coating resin or underfill resin 7 in the volume or space defined by the hybridized components 2 and 3, thereby ensuring the mechanical strength of the hybridized component and the resistance in particular of the microbumps 6 subsequent to the different stresses that may be encountered by the sensor implementing said hybridized component.

Said coating resin may consist of epoxy, but may also be selected from the group that includes acrylates and silicones.

To confine the coating resin in the inter-component space, the lower component, in the case in point the substrate 3, is coated with a fine layer of material of non-wettability as regards said substance, with the exception of the hybridization area.

A view from above is thus given in FIG. 3 of just the lower part of the component. The reference 8 shows in broken lines the perimeter of the component and the reference 12 the area of confinement of the resin.

According to the invention, an area for the deposition 9 of the required quantity of coating resin is also defined, again by implementing the covering with non-wettability as regards the coating resin. The confinement of said coating resin by the presence of the non-wettability covering for the whole periphery of the component, makes it certain that said coating resin will not migrate beyond the necessary surface, thereby allowing the prior art drawbacks to be overcome.

Because of said confinement of the coating resin, it becomes possible to increase the concentration of hybridized components on a single substrate since the problems related to the migration of said coating resin beyond the inter-component area are no longer encountered.

The covering with non-wettability as regards the coating resin is not very thick, typically between 10 and 500 nm, and is constituted for example by SiOC or C₄F₈.

Said covering is deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD) from a precursor, Octamethylcyclotetrasiloxane (OMCTS) in the case in point and by using a carrier gas such as argon or hydrogen.

Depending on the power of the plasma, the pressure, the initial precursor concentration and the duration of the plasma deposition, the thickness of the deposited layer and the stoichiometry thereof. SiO_(x1)C_(y1)H_(z1) can be controlled. The material SiO_(x1)C_(y1)H_(z1) is of formulation such that 0.3≦x1≦0.4 and 0.15≦y1≦0.2 and 0.4≦z1≦0.55.

Quite clearly, said deposition method and corresponding treatments may be applied more generally to other types of siloxanes of close formulation, such as for example tetraethylcyclotetrasiloxane (TNCTS).

Said material may further be treated and transformed on the surface into a material with the general formulation SiO_(x2)C_(y2)H_(z2), with x₂>>x₁, y₂<<y₁ and z₂<<z₁, or more straightforwardly SiO_(x), in which x tends towards 2 in order in particular to increase the value of its surface, and particularly polar, energy, and induce a very strong wettability of liquids (contact angle of about 5° in respect of water), and a very marked increase in adhesion and cellular development (moving from a generation time of 40 hours in respect of crude OMCTS to 20 hours in respect of the treated OMCTS).

The treatments used are either of the plasma type, namely helium plasma or oxygen plasmas O₂, SF₆+O₂, CHF₃+O₂ etc.), or of the UV type in ambient air or in an oxygen-enriched atmosphere.

Said treatments in respect of this type of deposition therefore have the particularity of making wettable as regards a substance an area initially non-wettable as regards said substance. In other words, the initial low wettability or non-wettability property of the deposition becomes reversible.

This feature can thus to advantage be implemented in order to improve the adhesion of the covering resins, also known as “glop-top” in the technical field under consideration, intended, in a known way, to promote and optimize the mechanical cohesion of the component obtained after hybridization.

Thus, initially (FIG. 8 a), the deposition of the localized anti-wetting coating, and for example SiO_(x1)C_(y1)H_(z1) controls by confinement the spread of the coating or underfill resin 5, 7 under the component or hybridized element 2, then in a second phase, the unit so obtained is subjected to the action of a plasma or ultra-violet radiation 21 (FIG. 8 b), tending to make said confinement covering wettable as regards a covering or coating or “glob-top” resin 20, and thereby improves its properties of adhesion with said resin, in order to promote an effective and durable coating of the component after hybridization (FIG. 8 c), and prevent the incipient cracking that is frequently observed on components obtained by so-called “flip chip” hybridization.

Said coating resin may be of the same type as the underfill resin, and for example, epoxy, silicones, acrylates etc.

The inventive method therefore also offers the possibility of easy localization by treating, using either mechanical masks or optical masks, areas of strong surface energy. This action means that droplets of various chemical species, from relatively non viscous liquids (water, biological molecules, oil) to viscous liquids (gum, resin, paint) can be localized on a pattern determined by the mask, and on the other hand, the adhesion of living cells (and the adsorption of biological molecules) can be localized in said areas.

According to an alternative of the invention shown in FIG. 4, the area of non-wettability 10 is modified by defining an additional surface 11, in the case in point opposite to the surface 9, and intended to collect any excess coating resin.

Said method may be implemented to advantage in respect of particular components and in particular to prevent the deposition or migration of the coating resin under the chip 2, and generally speaking under the active component.

An optoelectronic component has thus been shown in relation to FIGS. 5 and 6, wherein a laser 13 is hybridized on a substrate 3. The latter is bored with a through orifice 14, intended to let the laser beam through and to receive an optical fibre 15. Quite clearly, in the context of a VCSEL of this kind, the area of the chip located in line with the orifice 14 must of necessity be free from coating resin. To this end, on the periphery of the through orifice 15 is deposited a ring 16 of a material of non-wettability as regards the coating resin.

Another particular component implementing said coating method has also been shown in relation to FIG. 7. In the case in point, it is required that the adhesive securing the linear lens array 17 to a transparent substrate 18 does not come to disturb the optical function of said lenses 17 and does not affect the optical path of the laser beams coming from a linear laser array 19, hybridized on the other face of said transparent substrate 18. To prevent the adhesive from migrating and spreading over the optical paths, an anti-wetting covering is deposited, for example in the form of a peripheral seam as with the previously described example.

The full advantage of the invention can now be seen, in simplifying the means to be employed in coating the component hybridization area in order to guarantee the mechanical strength thereof. 

1. A method of coating the area of hybridization of a component constituted by two elements associated with one another by means of a soldering material, respectively a first and a second element, comprising depositing in proximity to said component a coating substance capable of filling by capillarity the volume defined between the hybridized elements of said component, method wherein on the periphery of the hybridization area on the lower element of said component is defined an area of non-wettability as regards said substance, wherein the non-wettability area is produced by deposition of a covering that is anti-wetting as regards said substance, obtained by PECVD, and defining on said first element, in line with the hybridization, a wetting surface for the coating substance,
 2. The coating method as claimed in claim 1, wherein the anti-wetting covering is constituted by SiOC or C₄F₈.
 3. The coating method as claimed in claim 1, wherein the area of non-wettability as regards the coating substance also defines a surface contiguous and in continuity with the wetting surface, intended to receive the deposition of said coating substance.
 4. The coating method as claimed claim 1, wherein the area of non-wettability as regards the coating substance defines a surface contiguous and in continuity with the wetting surface, intended to collect any excess coating substance.
 5. The coating method as claimed in claim 4, wherein the collecting surface of the excess coating substance is located opposite to the deposition surface of said substance relative to the hybridized component.
 6. The coating method as claimed in claim 1, wherein a surface of non-wettability as regards the coating substance is also defined on said first element, located in the hybridization area and in a particular area in which the presence of the coating substance is not required, said non-wettability surface also being obtained by deposition of a covering that is anti-wetting as regards said substance.
 7. A coating method as claimed in claim 1, wherein wettability properties are conferred afresh on the anti-wetting covering after the migration of the coating substance between the hybridized elements by subjection of the component obtained to the action of a plasma or to the action of an ultra-violet radiation.
 8. The coating method as claimed in claim 7, wherein after the plasma or ultra-violet radiation treatment, the component so obtained is coated with a coating resin.
 9. The coating method as claimed in claim 1, wherein the anti-wetting covering is constituted by SiO_(x1)C_(y1)H_(z1), wherein: 0.3≦x1≦0.4 0.15≦y1≦0.2 and 0.4≦z1≦0.55. 